1. Field of the Invention
The present invention relates to a video signal processor and, more particularly, to a luminance signal and a color signal separating circuit, and a noise reducing circuit for a video signal.
2. Description of the Related Art
FIG. 15 is a block diagram of the circuit of what is called a comb filter which is generally used for a VTR, a TV set, etc. In the NTSC broadcasting system, a color burst signal and a color signal are inverted before transmission for every horizontal line (about 63.5 .mu.s, hereinunder referred to as "1H"), as shown in FIG. 18. Such a comb filter is utilized as a filter for separating a luminance signal (Y signal) from a color signal (C signal). In a VTR, a comb filter is utilized as a filter for cancelling the crosstalk component in a color signal.
In FIG. 15, the reference numeral 100 represents a video signal. The video signal 100 is input to a band pass filter 1 (hereinunder referred to as "BPF"), for transmitting only a band of frequencies in which the color signal component exists, and a delay circuit 2. The output of the BPF 1 is input to a 1H-delay circuit 3 for delaying a signal by a 1H period, and the output of the 1H-delay circuit 3 is input to a subtractor 4. The subtractor 4 subtracts the output of the 1H-subtractor 3 from the output of the BPF 1. The output of the subtractor 4 is input to a subtractor 6 as a C signal through a gain adjuster 5. The subtractor 6 generates a Y signal by subtracting the output (C signal) of the gain adjuster 5 from the output of the delay circuit 2.
The operation of this comb filter will now be explained. The operations of the 1H-delay circuit 3 and the subtractor 4 will first be explained. Due to the above-described structure of the 1H-delay circuit 3 and the subtractor 4, the transmission function in this part is represented as follows: EQU H(z)=1-e.sup.-z .thrfore.H(.omega.)=1-cos .omega.T.sub.H +j sin .omega.T.sub.H (T.sub.H represnets a 1H.thrfore.H(f)=1-cos .sub.2.pi.fTh +j sin 2.pi.fT.sub.H
Accordingly, ##EQU1##
From this equation, it is obvious that the frequency characteristic is a function of a repeated reciprocal of T.sub.H. More specifically, the frequency characteristic resembles the shape of the teeth of a comb which has the maximum value when ##EQU2## (n is an integer) and the minimum value when ##EQU3##
In multiplexing a luminance signal and a color signal in the NTSC broadcasting system, the frequency-division multiplex system which utilizes frequency interleaving, as shown in FIG. 16, is adopted. Therefore, a color signal spectrum is distributed between every adjacent luminance signal spectra emitted at an interval ##EQU4## It is therefore possible to extract either the luminance signal spectrum or the color signal spectrum emitted at an interval of f.sub.h by using the above-described comb filter composed of the 1H-delay circuit 3 and the subtractor 4.
The BPF 1 is a filter for transmitting a band of frequencies in which a color signal exists (e.g., 3.58 MH.sub.z .+-.500 KH.sub.z in the NTSC system), as shown in FIG. 17. The band of frequencies which are passed through the BPF 1 is set at a frequency band having a frequency characteristic resembling the shape of the teeth of a comb. The delay circuit 2 is a filter for compensating for the propagation delay of the BPF 1, the subtractor 4 and the gain adjuster 5. By the delay circuit 2, the phase of a video signal is adjusted and the gain adjuster 5 adjusts the gain of the color signal C which is extracted in accordance with the frequency characteristic in the shape of the teeth of a comb and output from the subtractor 4. The subtractor 6 removes the color signal C so as to generate the luminance signal Y.
The above-described comb filter, however, deteriorates the vertical resolution because the operation is carried out in the vertical direction of the screen, which leads to the following defects in the picture quality.
For example, video signals for producing a picture shown in FIG. 20(a) are shown in FIG. 18. In FIG. 18, the video signals before and after the video signal for the horizontal line n-th H on which a red color changes to a white color, as shown in FIG. 20(a), are shown. The signals output before the signal for the n-th H transmit a color signal red and the signals for the n-th H and therebelow transmit a color signal white. When these signals are input to the video processing circuit shown in FIG. 15, the video processing circuit outputs the signals such as those shown in FIG. 19. That is, on the n-th H, unnecessary signals are contained in the Y signal and the C signal. When these signals are output on the TV screen, the picture shown in FIG. 20(b) is produced. In other words, a phenomenon called color edging (red sags from the boundary line between red and white to the region of white) is produced on the line on called dot disturbance (a signal of 3.58 MH.sub.z is mixed with a the screen on which the color changes, or a phenomenon luminance signal) is produced.
Such a deterioration in the picture quality is a critical defect in a VTR. This is because color edging is produced not only in the luminance signal and color signal separation in the interior of a VTR but also in the horizontal correlative noise cancellation in the crosstalk canceller in the VTR and in the luminance signal and color signal separation for the VTR output signal carried out again in a TV and, as a result, color edging is produced extending over several H's.
The luminance signal is also deteriorated in vertical resolution, as shown in FIG. 21, which shows the state of the picture in which the black bar is sagging and is cut at the n-th H. The output of the BPF 1 on the (n-1)th H only exists in the transient portions. When a video signal for the n-th H is input, no output of the BPF 1 exists. However, since the 1H-delay circuit 3 supplies the output of the BPF 1, the unnecessary C and Y signals are output. The C signal and Y signal output in spite of the absence of the output of the BPF 1 on the n-th H causes cross color and the deterioration in the vertical resolution, respectively. In the picture displayed on the basis of the Y signal, a black dot and a white peak appear, as shown in FIG. 21, which deteriorate the definition of the picture.